We describe here a sensitive tunable diode laser absorption spectrometer (TDLAS) which was employed for ambient measurements of formaldehyde (HCHO) during the 1993 Idaho Hill/Fritz Peak Photochemistry Experiment. This system incorporated many new features and approaches including a novel astigmatic Herriott sampling cell which achieves a 100-m pathlength in a 3-L volume. We also describe procedures and tests carried out to ensure high accuracy, including the verification of HCHO standards by means of four techniques. During the field campaign, ambient HCHO measurements were acquired with an average 1&sgr; measurement precision of 0.17 ppbv employing 1--5 min integration times. When combined with a maximum systematic uncertainty of 10%, ambient HCHO concentrations around 1.5 ppbv were measured with an average total (random plus systematic) 1&sgr; uncertainty of 15% during the field campaign. In the intervening 2 years since the field experiment, additional features have been implemented for continuous unattended operation and improved performance. Rapid background subtraction now routinely allows HCHO measurements to be acquired with replicate precisions of 0.040 to 0.056 ppbv employing a 5-min integration period. This corresponds to routine minimum detectable absorbances of 1.2 to 1.6¿10-6 in an actual mobile laboratory field environment.¿ 1997 American Geophysical Union